The present invention relates generally to packet switched digital telecommunication networks, and more particularly to improvements in fully integrated voice, data, and video (multimedia) communication services through the shared use of transmission and switching facilities in an integrated services network, including but not limited to networks such as those defined by the CCITT ISDN (Integrated Services Digital Network) and Broadband ISDN (B-ISDN) standards. The present invention provides for the coexistence and integration of 1.2 kilobits per second (kbps) to 2.045 megabits per second (mbps) applications with B-ISDN (&gt;2.048 mbps) applications in a true multimedia network.
In recent years, the International Telegraph and Telephone Consultative Committee (CCITT), a telecommunications industry international standards-setting group, established Study Group 18 to undertake cooperative planning of B-ISDNs. A principal aspect of B-ISDN is the support it would offer to multimedia traffic applications, in which a multiplicity of traffic component types including voice, data, and video are to be communicated through the network. Each traffic component type exhibits significantly different characteristics or attributes from the others, and may have different characteristics among the members of its own type or class. For example, pure data traffic components may be interactive data, local file transfer data, facsimile data, and so forth, which have different burst sizes, or "burstiness". Such different attributes create differences in the requirements imposed on the network and local equipment for efficient and effective handling of the traffic component types in the communication between sources and destinations of the traffic. For instance, isolated loss of voice packets may be tolerated in telephone communications because the listener can comprehend the overall tenor of the conversation despite these slight gaps. Although quality suffers, the "human ear" is quite forgiving in these circumstances. Delays between different voice packets, i.e., a change in the sequence of the packets from source to destination, however, is unacceptable. In contrast, transmission of data such as X.25 packets may not be adversely affected by delay among packets in transmission, but the loss of individual packets can prevent restoration of an entire message.
In 1988, CCITT Study Group 18 approved recommendation I.121 which identified Asynchronous Transfer Mode (ATM) as the target solution for implementing B-ISDNs. ATM is an asynchronous time division multiplexing technique employing fast packet switching which communicates user information through the network in fixed length packets (called "cells" in the ATM jargon) of 53 bytes each. One mission of the Study Group and its Working Party 8 has been to standardize B-ISDN user network interfaces, including one at 155 mbps and another at 600 mbps. The present focus of the industry, however, is on fast packet (broadband) switching products at 1.54 to 45 mbps. For multimedia networks, the ATM scheme advanced by Study Group 18 uses fixed size cells each of which is assigned to a single user or traffic component type. Depending on user requirements at a given time, considerable bandwidth may be unused because partially empty channels are being transmitted.
In U.S. Pat. No. 4,980,886 titled "Communication System Utilizing Dynamically Slotted Information" (the "'886 Patent"), assigned to the same assignee as the present application, S. Bernstein discloses a multimedia system in which each packet or frame has the same payload size, with a fixed number of slots assigned to users, and in which the slot assignments may be changed periodically to improve communication performance. These are composite frames, packing several users/traffic component types into each frame, rather than only one user per frame.
The invention disclosed in the '886 Patent departs from prior burst switching technology by distributing user payloads among the available slots in a multimedia frame based on the specified bandwidth requirements of each user. The slots, which constitute portions of the available bandwidth for each frame, are not necessarily occupied by respective users from start to finish of a transmission. Instead, each user is guaranteed a certain minimum amount of bandwidth and all users contend for any unused bandwidth in each frame, according to their individual needs. The sending side packet switch allocates bandwidth on a frame-by-frame basis, so that users may be moved from one slot to another or to several slots in mid-transmission (i.e., on a "per burst" basis).
In the invention of the '886 Patent, unused bandwidth is not locked out; if a particular user has nothing to send or is not using its minimum guaranteed bandwidth (total slot or slots), the respective slot or portion thereof is allocated to a user having need for it. As the circumstances change, the allocations change. The receiving side packet switch monitors the slots in each incoming frame to keep track of the arriving information (data, voice, video, and so forth) and its sources, and to dispatch the information to its proper destination. Thus, the invention of the '886 Patent provides an entirely controllable bandwidth in which users are assigned priority rights to particular slots, but, depending on each user's particular need for bandwidth, bursts or blocks of information are temporarily allocated to unused slots or unused space in slots on a frame-by-frame basis.